Methods of performing frame rate conversion by performing, on a film image, 2:3 pulldown to NTSC or conversion from NTSC to PAL and methods of performing such frame rate conversion with efficiency have been proposed to a large extent.
These methods are characterized in that the image length before conversion and the image length after conversion are the same because frame insertion or thinning-out is performed so that the number of frames is changed before and after conversion.
On the other hand, a method is known where the image length is different between before and after conversion because frame insertion or thinning-out is not performed and the number of frames is not changed between before and after conversion (frame rate conversion method where an image after conversion is either slow motion or fast-forward of an image before conversion).
For example, a system has been proposed that realizes a smooth 1/2.5 slow-motion playback by converting an image shot at a frame rate of 60 frames per second into an image of 24 frames per second.
Moreover, a variable-speed frame rate shooting camera has been proposed where the user can freely change the frame rate at the time of shooting.
Images shot by such a variable-speed frame rate shooting camera are recorded, for example, such that a scene is recorded at 60 frames per second and another scene, at 12 frames per second.
Moreover, a frame rate conversion system for a variable-speed frame rate shooting camera has been proposed.
For example, when the above-mentioned image is converted into an image of 24 frames per second, a scene recorded at 60 frames per second is converted into an image in a slow motion of 1/2.5 and a scene of 12 frames per second is converted into an image at double speed.
In such a frame rate conversion system, the way time codes are attached to the image after conversion is important.
Specifically, since frame rate conversion changes the number of frames that are displayed in one second, time code conversion is essential.
In the frame rate conversion such that the image length is different between before and after conversion, there is a possibility that not only the time codes of the lowest two digits (the frame number at the corresponding hour, minute, and second) but also the time codes of the other digits (hour, minute, and second) are changed.
For example, when conversion to an image of 24 frames per second is performed by a conversion method where an image length is the same between before and after conversion, a frame assigned a time code 01:15:24:30 in an image of 60 frames per second is assigned a time code 01:15:24:12 (three of every five frames are removed in the thinning-out).
Needless to say, when conversion is performed by a conversion method where an image length is different between before and after conversion, a time code such as 01:15:24:XX is not always assigned to the frame.
It takes much time to manually assign time codes so that overlapping is avoided while establishing correspondence with time codes before conversion.